Figure:
Amplitude responses of the
feed forwardcomb-filter
(diagrammed in Fig.2.23) with
and
,
, and
.
a) Linear amplitude scale. b) Decibel scale. The frequency axis goes
from 0 to the sampling rate (instead of only half the
sampling rate, which is more typical for real filters) in order to
display the fact that the number of notches is exactly
(as
opposed to ``
'').

This is plotted in Fig.2.25 for
,
, and
,
, and
.
When
, we get the simplified result

In this case, we obtain nulls, which are points
(frequencies) of zero gain in the amplitude response. Note that in
flangers, these nulls are moved slowly over time by
modulating the delay length
. Doing this smoothly requires
interpolated delay lines (see Chapter 4 and
Chapter 5).